The present invention relates to a hydraulic system for utility vehicles, in particular agricultural tractors, for supplying pressure medium consumers, such as hydraulic actuators, that are arranged inside and/or outside of the vehicle with a pressure medium, such as a hydraulic fluid, including a control pump or at least one fixed displacement pump, the pressure of which is controlled with respect to the load pressure of the pressure medium consumers and exceeds the load pressure by a predetermined control pressure differential, the system comprising means for producing two control pressure differentials of different values.
A hydraulic system of the type mentioned above is known from the firm magazine xe2x80x9cHydraulik in Theorie und Praxisxe2x80x9d, Robert Bosch GmbH, 2nd revised edition, Stuttgart 1995, page 269. The system serves to supply pressure medium consumers being controlled by directional valves by means of a fixed displacement pump. The system includes a pressure scale keeping the conveying pressure of the pump (called the xe2x80x9cpump pressurexe2x80x9d in the following) constantly at a pressure level exceeding the corresponding load pressure by a predetermined control pressure differential. The system further includes a switching valve by means of which different control pressure differentials are provided. The switching from one control pressure differential to the other control pressure differential is realized by the switching valve being controlled by the load pressure of the actuated pressure means consumer depending on the switching position of the switching valve. The system is designed and arranged in a way that in case of non-prevailing load pressure and the directional valves being switched to the neutral position, the low control pressure differential is attained, whereas in case of opened directional valves and prevailing load pressure, the higher control pressure differential is attained. In this way, power loss when none of the pressure medium consumers is actuated is reduced.
In utility vehicles, as for example in the above-mentioned agricultural tractors, the hydraulic system supplies a majority of pressure medium consumers. Some of these consumers, called primary pressure medium consumers, are supplied by pressure medium lines fixedly arranged inside the vehicle and that work without great losses of pressure. Other consumers, called auxiliary pressure medium consumers, arc arranged in tools of all different kinds that move along with the vehicle. In case of need, they are connected to the hydraulic system by hydraulic couplings, and they are supplied by the hydraulic system. Thus, substantial losses of pressure occur inside the hydraulic couplings and inside the pressure medium lines leading to the auxiliary pressure medium consumers. The control pressure differential is increased to compensate for these pressure losses. The increase of the control pressure differential results in greater power losses in the case where no pressure medium consumers or exclusively primary pressure medium consumers are to be supplied.
It is an object of the present invention to provide a hydraulic system of the type mentioned above having minimized power losses.
The object of the present invention is achieved by the fact that, in the case of prevailing load pressure of the auxiliary pressure medium consumers and a highest load pressure of the primary a pressure medium consumers being less than the sum of the load pressure of the auxiliary pressure medium consumers and the control pressure differential of auxiliary pressure medium consumers minus the control pressure differential of the primary pressure medium consumers, the control pressure differential is increased with respect to the control pressure differential in cases where the load pressure of the auxiliary pressure medium consumers does not prevail.
Due to the adaptation of the control pressure differential to the need of the primary pressure medium consumers and to the auxiliary pressure medium consumers, respectively, it is possible to supply the consumers with a lowest possible pressure, and thereby to the power losses of the pump as low as possible.
Further details and features of the present invention are mentioned in the claims.